Holy crap. Doesn't look like anchorworm to me.. does it have a forked tail?
If it's planaria, they can latch onto a fish now and then but usually don't stay there long..
From another website, on anchorworm:
Only the female copepod attaches to and burrows into a fish host. Mature females can be seen with the unaided eye, and are about one inch in length. Females have a slender, white, thread-like body, with paired egg sacs trailing from the posterior end. The bases of the fins are the preferred sites for attachment, which begin initially as a small hemorrhagic (bloody) spot. Over time, a raised nodule develops from which the body extends, usually lying against the fish's side. Less commonly, anchor worms will attach to the gills.
If it's a leech, a 2 teaspoon per gallon salt water bath for 15 mins will make it drop off your fish. Then I'd coppersafe the lot, fish included. End of problem.
If it's an anchorworm (they have forked tails.. leeches do not) that won't work, you'll need to pull it out - make sure to get all of the worm out, including the head. The sooner, the better. And then look up meds to rid the tank of them..
Last edited by Aus; 06-26-2012 at 10:28 AM.